Pex (and Stubs and Moles) should work in this setting. However, Pex and Moles will not run the code "hosted" in IIS, so you will have to mole the relevant dependencies.
We don't have any guidelines yet; it is on our to-do list to write a tutorial, similar to the one we have for SharePoint, that shows how to apply Pex on ASP.NET and MVC. But we haven't done it yet. If you gather some experience, it would be great if you could share it with us -- problems, but also what you found work well.
Thanks
Nikolai Tillmann -
Tell us how you use Pex